Polymer dispersed liquid crystal (PDLC) films, consisting of liquid crystal microdroplets dispersed in a polymer matrix, are potentially useful for solar energy control and other electro-optic applications, such as sunroofs, solar windows and information displays. Generally, these materials are formed by the incorporation of liquid crystals in a thermoplastic binder, or in a polymer matrix which has been cured using thermal, ultraviolet or electron-beam methods. The polymer dispersed liquid crystal films are typically sandwiched between a pair of transparent substrates. An electrically conductive transparent electrode is provided on each substrate, so that each of the transparent electrodes contacts the polymer matrix having the liquid crystals dispersed therein. Alternatively for applications where total reflectance is desired, these films may also be provided between one transparent and one reflective substrate, wherein each substrate has a corresponding transparent or reflective electrode. These polymer dispersed liquid crystal films can easily and reversibly be switched from an off-state which is cloudy, opaque, and light scattering, to an on-state which is essentially transparent. Most often this switching is accomplished by application of a suitable electrical voltage across the thickness of the film. However other methods for accomplishing this change in the transparency of the film include the application of heat or stress, or alternatively the application of a magnetic field across the thickness of that portion of the film where transparency is desired.
If the change in the transparency of the film is controlled by application of an appropriate electrical voltage and thereby an electrical potential is generated across the thickness of the film, it is necessary to make electrical contact to the polymer dispersed liquid crystal film. This is a particularly important, yet problematic, feature of these films and the devices incorporating these films.
Typically, the electrical contacts are connected to the transparent electrodes provided on each transparent substrate. However these transparent electrodes are extremely thin layers of either an appropriate electrically conductive oxide or metal; generally only about a few hundred Angstroms thick. They are therefore fragile both mechanically and chemically. Inadvertent mechanical abrasion and/or chemical etching during assembly of the contacts to the electrodes, may easily damage the films causing loss of electrical continuity between the contacts and the electrodes. Alternatively, inadvertent chemical oxidation during normal processing may result in increased electrical resistivity. Further, these fragile films may be easily damaged by the passage through them of large electrical currents, such as the electrical currents required in large area devices like window panels or when a high frequency electrical signal (i.e., above about several hundred Hertz) is used to enhance the electro-optic response. Therefore, it is desirable to provide a means for making the electrical contacts to the electrodes, which avoids these shortcomings.
Many techniques have been employed to make the necessary electrical contacts to the transparent electrodes, including self-sticking electrically conductive tapes such as formed from a metal; conductive adhesives such as adhesives loaded with electrically conductive particles; self-sticking unidirectional conductive composites; and conductive foams such as carbon-filled polyurethane and styrofoam. However, none of these techniques provide the desired level of reliability in use. These contact means fail fairly easily under any applied mechanical stress. They also tend to fail easily when large electrical currents are passed through them. Therefore they are not suitable for the activation of large devices having these polymer dispersed liquid crystal films since the current drawn by these films increases with the increased size of the film.
In addition, another frequent and significant problem which occurs during use of these conventional contact means is electrical arcing between components. Electrical arcing can be extremely detrimental since it often results in damage to the transparent electrode and consequent failure of the electrical contact. The conditions which promote electrical arcing between components are generally due to poor techniques for connecting the electrical contacts to the transparent electrodes. These assembly techniques frequently cause electrical shorts to develop, particularly when the contact means employed requires large pressures to establish electrical contact with the transparent conducting electrode. This is a requirement for many of these conventional self-sticking electrically conductive materials. Even when properly applied, it is difficult to apply the electrical contact uniformly along its entire length, which is adjacent with the electrode. As a result, frequent arcing occurs and eventually destroys the electrical contacts.
A further disadvantage associated with the use of these conventional electrical contact means is that, upon repeated bending of the polymer dispersed liquid crystal films, these types of aforementioned electrical contacts usually fail. Since one of the potential advantages of these types of polymer dispersed liquid crystal devices is their flexibility, it is preferred that the electrical contacts have a similar degree of flexibility.
Therefore, the preferred electrical contact means, which is employed to make connection to the transparent electrodes in these devices having these polymer dispersed liquid crystal films, should preferentially satisfy several attributes. First, the materials used to make the electrical contact should be chemically compatible with the electrode material, so as to alleviate the corrosion or oxidation of the thin layer of transparent electrode. Secondly, the assembly of the contact means should result in a consistently uniform and intimate bond between the contact means and the electrode, so as to eliminate any electrical shorts or arcing which may occur particularly when large currents are passed through the contacts such as in large area devices or when a high frequency signal is applied. Also, it is desirable that the contact means provide redundancy in the electrical connection so as to ensure that even multiple failures at the contact pad will not result in a complete loss of electrical contact.
In addition, the contact means should be able to tolerate large amplitude bending motions in order to survive subsequent processing and use of the polymer dispersed liquid crystal films, particularly when flexed or employed in tightly bent applications. The contact means should also have good overall durability and environmental stability, so as to enable widespread use of the devices containing these films. Also, the contact means should be capable of passing large currents without failure or electrical arcing. Lastly, other features which would also be desirable are design simplicity, ease and speed of fabrication, as well as low cost.
Accordingly, what is needed is a means for electrically contacting a polymer dispersed liquid crystal film which provides these desired features and which thereby alleviates the shortcomings associated with the use of conventional contact means.